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[Display omitted] • Key factors influencing the severity of traffic accidents in the four stages before and after the outbreak of the Covid-19 in Illinois, USA are determined. • Over the Covid-19, adapting regulatory measures to human perception factors and changes in customary travel times and accident-prone spatial segments can improve traffic resilience. • Recommendations are proposed for policy makers to improve the resilience of the transport sector in the event of public health crises. Transportation systems are critical lifelines and vulnerable to various disruptions, including unforeseen social events such as public health crises, and have far-reaching social impacts such as economic instability. This paper aims to determine the key factors influencing the severity of traffic accidents in four different stages during the pre- and the post Covid-19 pandemic in Illinois, USA. For this purpose, a Random Forest-based model is developed, which is combined with techniques of explainable machine learning. The results reveal that during the pandemic, human perceptual factors, notably increased air pressure, humidity and temperature, play an important role in accident severity. This suggests that alleviating driver anxiety, caused by these factors, may be more effective in curbing crash severity than conventional road condition improvements. Further analysis shows that the pandemic leads notable shifts in residents' daily travel time and accident-prone spatial segments, indicating the need for increased regulatory measures. Our findings provide new insights for policy makers seeking to improve transportation resilience during disruptive events. [ABSTRACT FROM AUTHOR]